g_io_channel_get_line_term: add (out) annotation

Fixes #2895 (closed)

I did not add the suggested (optional) annotation because it could be misinterpreted to mean that the parameter points to an optional gint32, instead of being an optional pointer to a gint32. An example of a case where (out) (optional) seems to mean "optional pointer to an optional value" is here.

In any case, the fact that the pointer can be NULL does not seem useful information for language binding generators anyway. I expect that most language bindings would always provide a non-NULL pointer.